Many semiconductor devices generate heat that, if not dissipated, can raise the temperature of the device sufficiently to damage it, causing abnormal operation or complete failure. It is therefore common practice to attach the semiconductor device to some form of heat dissipator, which functions to absorb heat from the device and transfers the heat to the surrounding atmosphere.
Typically, a semiconductor device is furnished with a heat-transmitting surface that can be attached by a mechanical fastener, such as a machine screw, to a heat dissipator. One commonly used heat dissipator is a heat sink. For convenience, the term xe2x80x9cheat sinkxe2x80x9d is referenced herein, though it should be understood that other types of heat dissipators, such as heat pipes, are equally applicable. The assembled semiconductor device and heat dissipator are then attached to a support, such as a printed circuit board. Electrical leads emanating from the semiconductor device are attached, usually by soldering, to connection pads located on the printed circuit board. The mechanical fastener attaches the heat transmitting surface to the heat sink, causing sufficient contact pressure to be created between the heat transmitting surface of the semiconductor device and the heat sink for there to be good heat conduction. The mechanical fasteners are installed through holes in flanges on the power semiconductor devices and are then threaded into holes in the heat sinks.
In many applications, power semiconductive devices experience significant thermal cycling. Such uses include, for example, the consumption of power in cellular base stations. Power amplifiers, one type of power semiconductive device or module, are used in the radio units (TRXs) of cellular base stations. Repeated heating and cooling of the mounting assembly, in combination with different thermal expansion characteristics of the individual components, can cause loosening of screws, stress relaxation of small screw threads, reduction in contact pressure due to thermal expansion of fastening devices such as screws, or a combination thereof. Accordingly, the force exerted by screws can decrease over time and as this occurs, the cooling effectiveness of the assembly is reduced. Thereafter, thermal cycles increase in magnitude, and can eventually lead to overheating and failure of the power semiconductive device, which is one of the primary field failure modes of TRXs. Mechanical fasteners can require time-consuming assembly procedures and tight geometric tolerances, particularly if several devices are to be mounted in sequence. Also, since the semiconductor must be electrically insulated from the heat dissipator, the fastener used to attach the semiconductive device mounting tab or flange in many uses must be insulated.
Alternative arrangements to mechanical fasteners for mounting heat-generating electrical components to heat sinks include heat sinks that resiliently grip the semiconductor device. This technique eliminates the need for mechanical fasteners, such as screws, which require accurate alignment and time consuming assembly methods. Many of these alternative arrangements require modifications to the structural design of the heat sink in order to be able to accommodate, for example, resilient clips or springs that couple the electrical device and the heat sink. Clip-on heat sinks are, in general, limited because of their low thermal mass that can be effectively coupled to the component. Springs are also used in some arrangements, but in general these arrangements are negatively affected by thermal cycling, thermal expansion, or both.
The present invention is directed to a mounting assembly for a heat-generating component, such as a power semiconductive module, using a clamping load. A clamp body is provided that may be fastened to a cover that protects the printed circuit board and module, or to another relatively fixed surface spaced from the module. The clamp body has two spaced, substantially parallel, raised edges extending away from the cover toward the module. A clamp spring is a beam that is compressed so that its ends abut the raised edges of the clamp body, and the installed spring has a generally convex curvature relative to the position of the module. At the apex of the spring""s curvature, which occurs at the area along the spring farthest from the clamp body, the spring contacts and applies force to the module on the surface of the module opposite the module""s heat-conducting surface. This force is transferred to the clamp body through the two ends of the spring. When the assembly experiences thermal expansion, the expansion of the spring causes the clamping force to increase.
In further embodiments, the clamp body is made of a material with a low coefficient of thermal expansion, and the clamp spring is made of a material with a relatively high coefficient of thermal expansion. Such a differential may provide greater relative expansion of the spring and accordingly a higher clamping force.
Additional various embodiments provide different configurations of the clamp body, including inward extensions on the raised edges, a two-part body, and outward extensions that appropriately space the clamp body from walls attached to the cover. Specific applications include but are not limited to the use of the present invention to mount a power amplifier in a radio unit (TRX) of a base station. A clamp, including the clamp body and clamp spring, may also be used in other embodiments and in other applications. The present invention also includes embodiments for mounting a plurality of modules using a clamping force. The spring may be constructed with more than one layer of material, with differing materials having different coefficients of expansion, altering the performance of the spring.
A method of assembling the module mount is provided. The module is placed in contact with a heat sink, and a clamp body is fastened to a cover. A clamp spring is inserted in the body so that the spring ends abut raised edges of the body, causing the spring to have a convex curvature relative to the module. The apex of the spring is brought into contact with the module, and the cover is fastened in position such that the spring biases the module against the heat sink, with the force exerted by the spring increasing with an increase in temperature.
Another method is provided for biasing the module against the heat sink. A spring beam has two ends that each abut a respective substantially fixed points to make a spring of convex curvature relative to the module, and the central apex portion of the spring is in contact with and applies force to the module. The force exerted increases with an increase in temperature to provide additional clamping force.